Conventional MLCs have generally been designed with the ability to shape the maximal radiation filed size that a medical linear accelerator (MLA) can produce in one instantaneous static exposure. Maximal field size is typically 40×40 cm2 in the patient plane produced by a radiation source 100 cm distant. However, conventional MLCs face a design dilemma in that the MLC leaf tips cannot be extended far from their carriage boxes. A relatively large cantilevered extension may increase the variance in the leaf tip lateral position, thus increasing leaf gap leakage and possibility of colliding with adjacent leaves.
To solve this dilemma, some conventional MLCs make leaf tails very long so that the leaf tips can be cantilevered with more precision from a non-moving (carriageless) leaf box. This leads to expensive heavy leaves and bulky, large collimators. Field shaping flexibility is limited by the leaf tip over-travel beyond the centerline. However, the leaf over-travel is generally not enough to block the entire filed. Some conventional MLCs put shorter leaves in a moving carriage box on reach side. Leaf cost and cover diameter are more acceptable but field shaping flexibility depend on the instantaneous leaf reach, or the projected distance between the most extended and the most retracted leaf tip on the same carriage box.
Dynamic treatment techniques such as intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) are rapidly being adopted in radiation treatment. These techniques change the paradigm of MLC use from Cerrobend block replacement to that of achieving complex 2D or 3D dose distribution goals. IMRT techniques produce good dose distributions from superposition of several dynamic ports. IMRT works well with carriage MLCs since the sliding window can be synchronized to not require large leaf reach. VMAT techniques are also successful for good dose distribution, especially for short treatment times. However, VMAT techniques are inherently limited by MLCs which have not been designed with such complex techniques in mind. Leaf reach and carriage position limitations have not suited VMAT optimally. For instance, in a conventional MLC, the instantaneously available fully independent field shaping area is only 15×40 cm2 because the leaf reach is only 15 cm. Independent field shaping area of 30×40 cm2 is theoretically available, but the dependencies of leaf travel range on leaf reach and carriage position limit instantaneous shaping ability so significantly that only slightly more than 15×40 cm2 is typically practical.
Accordingly, there is a need for MLCs that can overcome the problems associated with conventional MLCs. There is a need for MLCs that can cover most clinical treatments as well as or better than conventional MLCs but by using fewer leaves for less cost and higher reliability.